Substituted 2-(2-pyridinylmethyl sulphinyl)-1H-benzimidazoles such as omeprazole, lansoprazole, pantoprazole and rabeprazole are known as gastric acid secretion inhibitors. Omeprazole, chemically known as 5-methoxy-2-[(4-methoxy-3,5-dimethyl-2-pyridinyl)methylsulphinyl-1H-benzimidazole) which is useful as an antiulcer agent.
These structurally related sulphoxide compounds have a stereogenic center at the sulphur atom and thus exist as enantiomers.
There are several methods known in the prior art, which discloses the asymmetric oxidation of prochiral sulfide.
U.S. Pat. No. 5,948,789 and Eur. J. Biochem., 166 (1987), 453, describes enantioselective synthesis of substituted sulphoxide. In this process, a prochiral sulphide is oxidized into the corresponding sulphoxide either as a single enantiomer or in an enantiomerically enriched form using oxidizing agent in the presence of chiral titanium complex and in presence of base and organic solvent.
Similarly, WO2003089408 describes an enantioselective catalytic oxidation of sulfide with an oxidizing agent in an organic solvent and base and in the presence of titanium or vanadium complex with a chiral monodentate ligand.
WO2005054228 describes an enantioselective process for the preparation of substituted benzimidazole by asymmetrically oxidizing substituted prochiral sulphide preferably halo or nitro substituted, in an organic solvent and base with an oxidizing agent and a chiral titanium complex into the corresponding sulphoxide.
WO2005080374 describes enantioselective synthesis of sulphoxide compound either as a single enantiomer or in an enantiomerically enriched form by oxidizing prochiral sulphoxide, with an oxidizing agent in an organic solvent at a temperature between 20-40° C. and in the presence of chiral titanium complex.
WO2006040635 describes synthesis of substituted sulphoxide by oxidizing prochiral sulphoxide in presence of chiral titanium metal complex and a base in the absence of an organic solvent.
The above mentioned prior art references discloses the process of oxidation which is costly and not an environmental friendly. The other disadvantage is requirement of strict reaction condition during the oxidation and formation of sulphone by-product, makes the process industrially uneconomical.
Therefore, there is an increasing need for developing an economically viable and eco-friendly process for the preparation of sulphoxide either as a single enantiomer or in an enantiomerically enriched form, which avoids the use of organic solvent and base, the product is free from sulfone by-product and the process of preparation should to be cost effective and high yielding.